1. Technical Field
This invention relates to a method of effectively selecting live seeds (those which are able to germinate) of sugar beet out of a mixture thereof with dead seeds (those which are unable to germinate), thereby enhancing the germination rate of sown seeds.
2. Description of the Prior Art
The fruit of sugar beet (the material for the production of beet sugar) is of two types: polyembryonic and monoembryonic. The latter type is more commonly used because of the better adaptability to sowing machines and less labor required for subsequent thinning work.
Sugar beet is a biennial plant. Vegetative propagation progresses to grow its roots in the first year after sowing, and reproductive propagation follows in the second year, resulting in florescence and fructification. The fruit of sugar beet is characterized in that each piece has a seed cap closely fitted onto the outer shell.
The harvested fruit is usually freed from impurities, made even in grain size, and stored as seeds to be sown in the next season. Not all of the seeds thus selected germinate when sown, the rate of germination usually being about 90%. This indicates that the stored seeds are a mixture of live and dead seeds. The rate of germination could be enhanced if the live seeds are selected out of the mixture and only the selected seeds are sown.
No effective method is presently available to sort live seeds of sugar beet from dead seeds, but many sorting methods have been proposed for seeds in general. For example, U.S. Pat. No. 3,992,814, patented Nov. 23, 1976, discloses a technique, in which seeds are incubated to a state just before the start of germination, the incubated seeds are aligned on the surface of a liquid, and a downward stream from the surface is created, thus forcing the live seeds to descend. U.S. Pat. No. 4,467,560, patented Aug. 28, 1984, teaches a method in which seeds soaked with water are allowed to stand until just before the start of germination, followed by drying, and live seeds are sorted from dead seeds by utilizing the difference in the degree of water absorption. The present inventor has found that, if the fruit of sugar beet is immersed in water until just before the roots begin to come out, live seeds expand as a result of water absorption to make the closely fitted seed caps readily releasable. Based on these findings, I established a technique for selecting live seeds out of dead seeds, and applied for Japanese patent application No. 235,331, filed Oct. 23, 1985, not laid open.
When the techniques described in the above-mentioned U.S. Pat. Nos. 3,992,814 and 4,467,560 (both relying solely upon weight differences for sorting) are applied to the sugar beet seeds, the sorting accuracy is very low because the weight fluctuation among individual fruits is great and because the outer shell occupies about 50 to 60% of the total fruit (on dry basis), thus making the percentage of water absorption in the shell predominate in both live and dead seeds. In the method described in Japanese patent application No. 235,331, live seeds (fruit) of sugar beet can be effectively distinguished from dead seeds by the release of seed caps, but it leaves much room for improvement because the conditions that cause easy release of the seed caps are limited to a very narrow range.
It is generally accepted that the optimum condition for harvesting high-quality sugar beet at high yields is even growing of 70,000 plants per hectare. When seeds are sown in numbers that correspond to this density, there will be missing plants because of the dead seeds involved. If an excessive number of seeds are sown to make up these wanted plants, a great deal of labor is needed for subsequent thinning work. In Japan, a culture technique using paper tubes known under the name of Paperpot (registered tradename) is popular for seedling culture and transplantation of sugar beet (now occupying 98% of its total planting). In this case, too, the highest production efficiency can be expected only when all the sown seeds germinate. Presence of dead seeds requires additional work to manually or mechanically select and remove the vacant paper tubes prior to transplantation. When this work is omitted, additional seedlings have to be set out after transplantation. The increase in cost caused by the vacant paper tubes is also enormous. Thus, there has been a great demand for a method of selecting live seeds of sugar beet that will surely germinate and assure high-quality products at high yields.